One challenge facing the delivery of radiation to treat pathological anatomies such as tumors or lesions is identifying the location of the target (i.e. tumor location within a patient). The most common technique currently used to identify and target a tumor location for treatment involves a diagnostic X-ray or fluoroscopy system to image the patient's body to detect the position of the tumor. This technique assumes that the tumor is stationary. Even if a patient is kept motionless, radiation treatment requires additional methods to account for movement due to respiration, in particular when treating a tumor located near the lungs. Breath hold and respiratory gating are two conventional methods used to compensate for target movement during respiration while a patient is receiving conventional radiation treatments.
Breath hold requires the patient to hold his or her breath at the same point in the breathing cycle and only treats the tumor when the tumor is stationary. A respirometer is often used to measure the tidal volume and ensure the breath is being held at the same location in the breathing cycle during each irradiation. Such a breath hold method takes longer than a standard treatment and often requires training the patient to hold his or her breath in a repeatable manner.
Respiratory gating is the process of turning on the radiation beam as a function of a patient's breathing cycle. When using a respiratory gating technique, treatment is synchronized to the individual's breathing pattern, limiting the radiation beam delivery to only one specific part of the breathing cycle and targeting the tumor only when it is in the optimum range. Such a respiratory gating method requires the patient to have many sessions of training and many days of practice to breathe in the same manner for long periods of time. A system implementing the respiratory gating method may also require healthy tissue to be irradiated before and after the tumor passes into view to ensure complete coverage of the tumor.
Attempts have been made to avoid the burdens placed on a patient from breath hold and respiratory gating techniques. Some methods for tracking the movement of a tumor or other target use imaging devices to capture the internal structure of a patient's body. One imaging modality that is commonly used in medical applications is ultrasound. Ultrasound systems create images of internal structure by detecting reflection signatures resulting from the propagation of high-frequency sound waves into the internal structure.
Conventional ultrasound systems are not suitable for use in target tracking applications because the imaging field of such systems is typically small, so that tissue movement affecting the imaged area is more likely to move a target out of the imaging field. Furthermore, repositioning of the ultrasound transducer to maintain image quality may require intervention by an operator whose presence in a treatment room may be disruptive, particularly during a treatment session.